Background
In MySQL 5.5, resuming a broken replication setup required you to determine the last binary log file and position, which are distinct on nodes if binary logging is enabled. If the MySQL master fails, replication breaks and the slave will need to switch to another master. You will need to promote the most updated slave node to be a master, and manually determine a new binary log file and position of the last transaction executed by the slave. Another option is to dump the data from the new master node, restore it on slave and start replication with the new master node. These options are of course doable, but not very practical in production.
How GTID Solves the Problem
GTID (Global Transaction Identifier) provides a better transactions mapping across nodes. In MySQL 5.5. or before, Replication works in such a way that all nodes will generate different binlog files. Binlog events are the same and in the same order, but binlog file offsets may vary. With GTID, slaves can see a unique transaction coming in from several masters and this can easily be mapped into the slave execution list if it needs to restart or resume replication.
Every transaction has a unique identifier which identifies it in the same way on every server. It’s not important anymore in which binary log position a transaction was recorded, all you need to know is the GTID: ‘966073f3-b6a4-11e4-af2c-080027880ca6:4’. GTID is built from two parts - the unique identifier of a server where a transaction was first executed, and a sequence number. In the above example, we can see that the transaction was executed by the server with server_uuid of ‘966073f3-b6a4-11e4-af2c-080027880ca6’ and it’s 4th transaction executed there. This information is enough to perform complex topology changes - MySQL knows which transactions have been executed and therefore it knows which transactions need to be executed next. Forget about binary logs, it’s now all in the GTID.
All necessary information for synchronizing with the master can be obtained directly from the replication stream. When you are using GTIDs for replication, you do not need to include MASTER_LOG_FILE or MASTER_LOG_POS options in the CHANGE MASTER TO statement; instead, it is necessary only to enable the MASTER_AUTO_POSITION option.
GTIDs (global transaction identifiers) - Transaction-based Replication
GTID (global transaction identifier) 即全局事务 ID，一个事务对应一个 GTID，保证了在每个在主库上提交的事务在集群中有一个唯一的 ID。
When using GTIDs, each transaction can be identified and tracked as it is committed on the originating server and applied by any replicas; this means that it is not necessary when using GTIDs to refer to log files or positions within those files when starting a new replica or failing over to a new source, which greatly simplifies these tasks. Because GTID-based replication is completely transaction-based, it is simple to determine whether sources and replicas are consistent; as long as all transactions committed on a source are also committed on a replica, consistency between the two is guaranteed.
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